The advent of high-exponent zinc oxide type varistors permits voltage surge arresters containing the varistors to be designed without the need for series gaps. The term "exponent" as used herein refers to the value of the current-voltage exponent "n" of the voltage in the current-voltage relationship for a nonlinear resistor given by the expression I=kV.sup.n where I represents the current through the varistor, K represents a constant and V represents the voltage across the varistor.
Earlier varistor material, such as silicon carbide had an exponent of about 4 to 5 which was too low an exponent to allow the varistor to be continuously and directly connected from line to ground.
Thus, if the varistor impedance was chosen to limit the discharge voltage to a practical value at a current of 10,000 amperes during the discharge of lightning surge, then at normal operating voltage the current would be sufficiently high to cause overheating and finally failure by thermal runaway. For this reason series spark gaps were used with the low-exponent devices, providing an open circuit between the varistor and ground during normal operating conditions. In order for current to flow through the low-exponent varistor devices an overvoltage surge condition sufficient to spark over the series gap had to occur before the varistors could become conductive to ground.
The high-exponent zinc oxide type varistors have an exponent of about 25 or even higher over the operating range of interest. A consequence of this high an exponent is that a varistor of this material which is designed to be stable, that is, to continuously withstand normal system operating voltage will, when subjected to a lightning current discharge of say 10,000 amperes, limit the voltage to a level which is only about 10% higher than the present protective level of the best arresters using the silicon carbide valve elements. In other words, the use of zinc oxide varistor material allows one to make a voltage surge arrester which has no spark gaps whatsoever and which can provide protection to within 10% of that provided by modern conventional arresters employing silicon carbide varistors.
In order to provide protective characteristics at least equal to or better than present day arresters it is necessary to consider bypassing (or removing from the discharge circuit) some 10% or more of the total varistor material during the higher current over-voltage discharges such as for example those caused by high-current lightning discharges. This can be done by providing shunt gaps in parallel with about 10% of the series-connected zinc oxide varistor elements and by causing these shunt gaps to spark over when the arrester discharge current reaches a level of some few hundreds of amperes during operation from an overvoltage condition.
The use of zinc oxide varistors and parallel shunt spark gaps is disclosed in U.S. patent application Ser. No. 805,737 filed June 13, 1977, now abandoned, and Swedish Pat. No. 7209436-0 filed Aug. 18, 1972. The description of low-exponent varistors having parallel spark gaps is described in U.S. Pat. No. 3,320,482 issued May 16, 1967. Said U.S. and Swedish patents and said U.S. patent application are incorporated herein by way of reference.
It is desirable that the arrester voltage level at which the shunt gaps sparkover be very accurately controlled. That is, it is imperative that they spark over before the arrester voltage reaches a level which exceeds the designed protective level. They should not spark over until absolutely necessary in order that the arrester is better able to withstand sustained system overvoltages above the normal level, but below the protective level, without damage.
The purpose of this invention is to provide an arrester containing high-exponent zinc oxide varistors and shunt gaps paralleling some portion of the zinc oxide varistors wherein the shunt gap sparkover voltage breakdown characteristics are accurately determined.